1. Field of the Invention
This invention relates to a clamp device for a rotational output shaft, and more particularly to a rotary indexing device for a rotational output shaft for rotary indexing the output shaft which is driven to rotate without applying unnecessary loads in the axial direction to thereby prevent axial displacement thereof, but applying a uniform pressure in the radial direction. This invention is applicable to the clamp mechanism for a cutter table or an indexing device of an NC lathe connected to an output shaft of a motor.
2. Discussion of the Background
As a clamp device for rotational shaft of this type, there has been known in the prior art a rotation indexing device of direct drive type which is incorporated within a clamp device as is disclosed in Japanese Utility Model Application Laid Open No. 121636/1989. In the device, a motor stator is fixed inside of a housing and a motor rotor is provided coaxially with the motor stator. The motor rotor is integrally fixed with an output shaft which extends in the axial direction and is journaled via bearings onto the housing in a freely axially rotatable manner. The motor stator, the motor rotor, the output shaft and the bearings form a rotational shaft which directly drives and rotates an indexing table. The rotational shaft is suspended at a predetermined position while the rotational angle thereof is being detected by a rotation detector and is held by a clamp mechanism at the position.
The clamp mechanism includes a cylinder which has a flange extending in the radial direction and is attached to the housing in an axially freely slidable manner, a piston which is provided to oppose the cylinder and is axially slidable so as to freely attach to/detach from the rear face of an indexing table, and a pressurizing chamber interposed between opposing planes of the cylinder and of the piston.
For clamping, when a pressurizing fluid such as the compressed air and oil is fed to the pressurizing chamber, the cylinder and the piston are slided to oppose each other in the direction to thrust the rotational shaft so as to intensely press the output shaft on the upper surface thereof by the cylinder for putting brakes on its rotation as well as to intensely press the rear surface of the indexing table by the piston to apply a force opposing the force applied on said output shaft in the thrusting direction. The thrusting force applied on the indexing table is thereby offset to prevent the indexing table from being displaced axially.
However, the prior art clamp device for rotational shaft is not quite satisfactory. Because axial rotations are restricted by applying load on the output shaft in the thrusting direction for clamping, it is necessary to apply counterforce on the indexing table in the opposite direction to prevent axial displacement of the table. Moreover, the braking force and the counterforce should be balanced. This disadvantageously makes the structure complicated, the size bulky and the weight heavy. As the restricting or breaking force to be applied on the output shaft is directed in the thrusting direction, the area for receiving the restricting force becomes extremely limited, which in turn increases the pressure in a unit area.